This invention relates generally to an electrical circuit control device and more particularly to fluid pressure actuated control device and a method of assembling such a control device.
Fluid pressure sensitive control devices typically include a housing having an opening for the communication of fluid pressure into the housing and a diaphragm which moves in response to the fluid pressure. A two position, bistable, snap-acting disc in the housing is movable, for example, from a generally convex configuration to a generally concave configuration upon application of sufficient pressure to the snap-disc from the diaphragm for actuating a switch in the housing to open or to close an electrical circuit to which the control device is attached. The snap-disc moves back to its convex configuration when the pressure applied by the diaphragm falls below a certain predetermined value. Control devices of the type to which this invention generally relates are used in automotive air conditioning systems to control the energization and deenergization of a clutch actuated compressor in response to a preselected low and high value of fluid pressure measured at a preselected point in the system, such as in an accumulator in the system. Examples of control devices of the same general type as disclosed herein are disclosed in Poling, U.S. Pat. No. 4,200,776, and Johnson, U.S. Pat. No. 4,464,551, which are incorporated herein by reference.
The control device is constructed so that the switch is either opened or closed by action of the snap-disc upon the detection of a predetermined level of pressure in the fluid system. In either case, the control device should be set so that the switch point, that is, the instant when the switch first makes or breaks contact to shut or open the electrical circuit occurs when the snap-disc is between its convex and concave configurations. Should the switch point be too near the convex or concave configuration of the snap-disc, the switch tends to oscillate between its open and closed positions because of small movements of the snap-disc caused by a slow pressure build-up (or relief) in the control device. Most signficantly, operating temperature conditions encountered by the control device and wear of parts over the life of the control device causes the switch point to drift toward the concave orientation of the snap-disc. Therefore, it is necessary to set the switch point nearer the convex position of the snap-disc to allow for this drift.
In production of control devices, properly setting the switch point is difficult because of the variations in component part sizes naturally arising from manufacturing tolerances for those parts. Presently, the switch point is set by attempting to hold part tolerances within limits which will result in the switch point being in one of an acceptable range of positions between the convex and concave configurations of the snap-disc. In some existing control devices, a one-way adjustment of the switch point can be made by fixing a pin in a position to engage a movable switch blade of the switch for applying a force to the switch blade in a direction opposite the force transmitted to the switch blade from the snap-disc. The application of this force by the pin adjusts the location of the switch point. Setting the switch point is a delicate procedure involving only a small range of appropriate positions of the pin. An important disadvantage of this approach is that the pin can only be moved in one direction, toward the switch blade. Thus, an overcorrection of the switch point cannot be remedied. In addition, this approach causes the switch blade to be subject to increased stress, thereby reducing its operating life.
Control devices must have the appropriate connection to the particular electrical control circuit to which they are to be attached. While the interior workings of the control device may be the same for various types of control circuits, changing the connector portion of the device requires a considerable expenditure of time and money because the housing is formed as one piece. Moreover, prior control devices do not allow the device to be tested prior to complete assembly. Thus, a defective diaphragm is not discovered until the device has been completely assembled, requiring that the device either be disassembled or discarded in its entirety.
Further difficulties in assembly of existing control devices occur because the switch is assembled as two separate pieces, one of which is a movable switch blade and the other of which is a stationary contact support having a contact engageable by a contact on the switch blade to close the switch. Although separation of the switch blade and the contact support is necessary in the device so that completion of the electric circuit is made only when the contact on the switch blade engages the contact on the contact support, the additional separate parts complicates and slows down assembly of the control device. In addition, proper alignment of the contact on the stationary contact support and the contact on the switch blade is difficult to attain.